Net losses, after factoring in other securities gains, are expected to exceed $800 million by the end of the second quarter. And losses could increase depending on market conditions and the bank's actions moving forward, CEO Jamie Dimon said.

Dimon, speaking to analysts and reporters on a conference call, said the losses were caused by "errors," "sloppiness" and "bad judgment."

"This was a unique thing we did," Dimon said. "Obviously it had a lot of problems. It was a bad strategy. It became more complex, it was poorly managed."

Last month, rumors swirled around a JPMorgan employee based in London who had, according to the Wall Street Journal, been taking large positions in credit default swaps, the insurance-like bets that blew up in the 2008 financial crisis. The employee was said to work in the bank's Chief Investment Office.

Finally, Mayo asked Dimon what he should have paid more attention to in hindsight.

"Trading losses," Dimon said simply. "Newspapers."

Another analyst asked about the "terrible" timing of the loss, given the debate over the Volcker Rule.

That rule, a part of the Wall Street reform law passed in response to the financial crisis, aims to ban risky trading by banks for their own profit, a strategy sometimes referred to as proprietary trading.

"It's very unfortunate ... it plays right into the hands of pundits out there, but that's life," Dimon said. Earlier in the call, he said: "It didn't violate the Volcker rule; it violated the Dimon principle."

But Dimon was quick to defend JPMorgan's overall track record: "When you look at all the things we've done, we've been very careful. And we've been quite successful. This obviously wasn't."

Still, the loss could weigh heavily on the bank's net profits, especially if losses in the CIO unit accelerate, something Dimon said is quite possible.